1. Field of the Invention
The present invention relates to an optical pick-up capable of recording and reproducing data on and from discs having different thickness, and more particularly to a hologram optical pick-up capable of recording and reproducing data on and from any kind of disc by using two laser sources emitting laser beams having different wavelengths, thereby improving efficiency of the laser beams.
2. Prior Arts
A conventional compact disc is well known as a recordable and reproducible optical medium. The compact disc generally is a substrate having a thickness of 1.2 mm and has a reflecting surface as a recording layer. Information such as a sound, a character, a graphic, and the like, are recorded in a form of pits on the reflecting surface. A digital video disc with a substrate having a thickness of 0.6 mm also has been developed as a recordable and reproducible optical medium. Information including image data are stored with high density in the digital video disc.
An optical pick-up for the digital video disc differs from an optical pick-up for the compact disc. The optical pick-up of the digital video disc includes a laser diode emitting laser beams having a short wavelength and an object lens having a high numerical aperture. Thereby, the optical pick-up of the digital video disc collects the laser beams and focuses the laser beams in a form of fine spots on a disc to reproduce data recorded with high density from the disc. On the other hand, recently, a phase changeable disc on which data are recorded only once and reproduced from has been developed. The phase changeable disc has the same thickness as that of the compact disc but has a different recording layer from the compact disc. The surface of the compact disc has pits formed as the data recording layer, whereas the phase changeable disc has phase changeable materials, by which a reflecting ratio of the laser beams during data recording is different from a reflecting ratio of the laser beams during data reproducing in a range of certain wavelength regions. The optical pick-up can record and reproduce data on the phase changeable disc and compact disc under the same environment.
As discs having different thicknesses or recording materials have been developed, optical pick-ups capable of using different discs has been required. Accordingly, an optical pick-up capable of recording and reproducing data on and from both a digital video disc and a compact disc has been provided. The optical pick-up capable of recording and reproducing the data on and from both the digital video disc and the compact disc generally uses a laser diode emitting laser beams having a short wavelength, for example, a wavelength of 650 nm, and can adjust a numerical aperture of an object lens with correspondence to the thickness of a disc to be reproduced. During data reproduction of the digital video disc, the optical pick-up focuses laser beams into a small spot on the digital video disc by using the object lens with a numerical aperture of 0.6, and during data reproduction of the compact disc, the optical pick-up makes the diameters of the laser beams, incident from the laser diode to the object lens, small or causes the laser beams to pass through a portion, for example, short axis portion at which the numerical aperture is 0.45, at which the numerical aperture is lower in the object lens. Thereby it is possible to compensate for an aberration of the laser beams. In the optical pick-up applying to both the digital video disc and the compact disc as described above, a surface emitting laser diode is used as a laser source, and a hologram is used as an element adjusting the numerical aperture of the object lens.
Referring to FIG. 1, a hologram plate 2 having first and second holograms 3 and 4 at both sides thereof is positioned in front of a laser source 1. A collimate lens 5 and an object lens 6 are disposed respectively in front of the hologram plate 2.
The laser source 1 emits laser beams. The laser beams pass through the hologram plate 2, which in turn are modulated into parallel beams by the collimate lens 5. Then, laser beams are incident into and focused by the object lens 6 on a digital video disc 7 and a compact disc 8. The laser beams are reflected by the digital video disc 7 and the compact disc 8 and pass again through the object lens 6 and the collimate lens 5. Then, the laser beams are diffracted by the holograms 3 and 4 and detected by first and second photo detectors 9 and 10.
During data reproduction of the digital video disc 7, the laser beams passing through the first hologram 3 are focused into a small spot on the digital video disc 7 by using the numerical aperture of the object lens 6. The reflected laser beams are diffracted by the first hologram 3 and received by the first photo detector 9. The first photo detector 9 changes intensity of the laser beams into electric signals. On the other hand, during data reproduction the compact disc 8, laser beams are made to pass through the second hologram 4 having a small area and the portion at which the numerical aperture of the object lens 6 is lower. Thereby, it is possible to compensate for the aberration of the laser beams generated due to the thickness difference between the digital video disc and the compact disc. Then, the laser beams are focused in a form of spots on the compact disc 8. The laser beams are reflected by the compact disc and diffracted by the second hologram 4, which in turn are detected by the second detector 10.
In the conventional optical pick-up as described above, however, since the optical pick-up uses the laser source emitting the laser beams which have the short wavelength as to focus the laser beams in the form of fine spots on the digital video disc, there is a disadvantage in that when the optical pick-up reproduces data from the phase changeable disc, the data can be destroyed due to properties of the phase changeable material.
Furthermore, in the conventional optical pick-up, the optical environment generally is adjusted for the digital video disc having a thinner thickness, thus there is a problem in that it is impossible to reproduce the data from the compact disc in good condition because during the reproduction of data from the compact disc, there is a loss of the laser beams which reach the photo detector, resulting in a deterioration of the reproducing signal.